1. Field of the Invention
The present invention relates, in a magnetoresistance effect element for detecting the magnetic field from a magnetic recording medium or the like as a signal, to a magnetic multilayered film capable of detecting a small magnetic field change as a greater electrical resistance change signal, and further relates to the magnetoresistance effect element using such a magnetic multilayered film and to a magnetoresistance device using such a magnetoresistance effect element.
2. Deescription of the Prior Art
Recently, there has been the developing for increasing the recording density in magnetic recording and, following this, magnetoresistance effect type heads (hereinafter referred to as MR heads) using magnetoresistance change have been actively developed. The MR heads are designed to detect external magnetic field signals on the basis of the variation in resistance of a detecting sensor portion made of a magnetic material. The MR heads have an advantage that a high output signal can be obtained in high recording density because the reproduced output signal does not depend on the relative velocity of the heads to the magnetic recording medium.
Ni.sub.80 Fe.sub.20 (Permalloy) is mainly used as the MR head material for high density magnetic recording at present. This material converts the change of a leaking magnetic field from a magnetic recording medium into the change of electrical resistance caused by an anisotropic magnetoresistance effect. However, a magnetoresistance change ratio .alpha.R/R (MR ratio) is in the range of 1 to 3% at most, and further, an MR ratio per unit magnetic field, that is, an MR sensitivity, is as small as about 0.05%/Oe, thereby resulting in reduction of the signal to noise ratio (S/N) for the recording density of no less than 3 GBPl (Giga Bit Per Inch.sup.2). Therefore, the MR head material with the higher MR sensitivity will be needed.
Attention has been recently paid to artificial lattices having the structure in which thin films of metal having a thickness of an atomic diameter order are periodically stacked, because their behavior is different from that of bulk metal. One of such artificial lattices is a magnetic multilayered film having ferromagnetic metal thin films and non-magnetic metal thin films alternately deposited on a substrate. Heretofore, magnetic multilayered films of Fe/Cr, Co/Cu, NiFe/Cu and other types have been known. Among these materials, the iron-chromium (Fe/Cr) type was reported to exhibit a magnetoresistance change which exceeds 40% at an extremely low temperature (4.2 K). However, this is not commercially applicable because the external magnetic field at which a maximum resistance change occurs (operating magnetic field ), is as high as ten to several tens of kilo-oersted. Additionally, there have been proposed artificial lattice magnetic multilayered films of Co/Ag or the like, which, however, also require high operating magnetic field. On the other hand, the NiFe/Cu type requires the operating magnetic field which is as low as no greater than 500 Oe, however, the MR sensitivity thereof is 0.5%/Oe at most.
Furthermore, in case of MR heads, etc., there are some cases where a laminate structure is complicated, and thermal treatments such as baking, curing, etc. of resist materials are required in a fabrication process, a flattening process, etc., so that thermal stability for a temperature of about 250.degree. C. is required for the MR materials. However, such a thermal treatment deteriorates the characteristics in the artificial lattice magnetic multilayered films.
Under these circumstances, a new structure which is called a spin valve has been proposed. In this structure, a non-magnetic layer is sandwiched by two kinds of magnetic layers of a free ferromagnetic layer and a pinned ferromagnetic layer made of NiFe or the like, and an antiferromagnetic layer made of FeMn or the like is further formed adjacent to the pinned ferromagnetic layer. In this case, since the FeMn layer and the adjacent pinned ferromagnetic layer are directly magnetic exchange-coupled to each other, the direction of the magnetic spin of the pinned ferromagnetic layer is fixed in the range of several tens to several hundreds Oe in magnetic field. On the other hand, the direction of the magnetic spin of the free ferromagnetic layer is freely varied by an external magnetic field. As a result, there can be achieved a magnetoresistance change ratio (MR ratio) of 2 to 5% in a small magnetic field range and the MR sensitivity of 0.6 to 1%. The following papers have been published relating thereto:
a. Physical Review B, 43 (1991) 1297
Si/Ta(5)/NiFe(6)/Cu(2)/NiFe(4.5)/FeMn(7)/Ta(5)
parenthesis represents film thickness (unit nm) of each layer, also applied hereinafter! is reported to exhibit that its MR ratio sharply rises up to 5.0% at an applied external magnetic field of 10 Oe.
b. Japanese Journal of Applied Physics, 32 (1993) L1441
The MR ratio is reported when the multilayer structure is adopted in the above paper a. In this muItilayer structure, the structure of NiFe(6)/Cu(2.5)/NiFe(4)/FeMn(5) is laminated so as to sandwich Cu therebetween.
c. Applied Physics Letters, vol. 65, 1183 (1994)
The magnitude and stability of unidirectional anisotropy are reported when a laminated structure is formed by FeMn, NiMn or the like, as an exchange-coupling film, and NiFe.
d. U.S. Pat. No. 4,949,039
It is described that a larger MR effect can be obtained by forming ferromagnetic thin films through a non-magnetic intermediate layer so as to be arranged in antiparallel to each other. In addition, it describes a structure in which an antiferromagnetic material is disposed adjacent to one of the ferromagnetic layers.
In the spin valve magnetic multi layered films shown in the foregoing papers, the MR ratio is lower than the structure of Fe/Cr, Co/Cu, Co/Ag, NiFe/Cu or the like. However, the MR curve varies sharply at the applied magnetic field no greater than several tens Oe, so that it is suitably usable as an MR head material for a recording density no less than 3 Gbit/inch.sup.2. Further, the spin valve film achieves the large MR effect by carrying out pinning with the antiferromagnetic layer adjacent to one of the two magnetic layers. Thus, the role of the antiferromagnetic layer is important and its reliability is extremely important. However, in case of FeMn mainly used at present, the Neel temperature is low, that is, about 150.degree. C., so that it is insufficient in practice. Further, since FeMn is liable to corrosion, there is a big problem in manufacturing process. Further, when a metal thin film having a low resistivity is used for the antiferromagnetic layer, a shunted portion of the current flowing in the element becomes large so that it is necessary to feed the large current for achieving a given output. As a result, the element is subjected to a problem of heat or the like, which is not preferable.
For solving the foregoing problem, it has been proposed to use, as an antiferromagnetic layer, an oxide thin film made of NiO, .alpha.-Fe.sub.2 O.sub.3 or the like having a high resistivity.
e. Japanese Laid-Open Patent Publication No. Hei 5-347013
A magnetic recording/reproducing device using a spin valve film is described. Particularly, it is disclosed that nickel oxide is used for an antiferromagnetic film.
However, when nickel oxide is used, thermal stability is up to about 220.degree. C., which is not sufficiently high. Further, there is a problem that the operating magnetic field margin is small due to a high coercive force of the pinned ferromagnetic layer.
f. Journal of Japan Applied Magnetics, 20, 365-368 (1996)
A spin valve GMR film was reported, wherein an .alpha.-Fe.sub.2 O.sub.3 film produced by an RF magnetron sputtering method was used as an antiferromagnetic film.
In this report, a magnetic multilayered film has a structure where .alpha.-Fe.sub.2 O.sub.3 /NiFe/Cu/NiFe are stacked in order from a substrate. In this magnetic multilayered film, a pinned ferromagnetic layer and a free ferromagnetic layer are both NiFe layers, while the antiferromagnetic layer is the .alpha.-Fe.sub.2 O.sub.3 layer. This magnetic multilayered film has the same film structure as that of the magnetic multilayered film exhibiting the spin valve type GMR effect. However, as shown in FIG. 2 of this report, this magnetic multilayered film shows the GMR characteristic due to the difference in coercive force which is different from the conventionally reported spin valve type GMR film. Specifically, the oxide antiferromagnetic layer .alpha.-Fe.sub.2 O.sub.3 is used for increasing a coercive force of the pinned ferromagnetic layer NiFe due to exchange coupling between the oxide antiferromagnetic layer .alpha.-Fe.sub.2 O.sub.3 and the adjacent pinned ferromagnetic layer NiFe. This function of the antiferromagnetic layer is different from that of the conventional spin valve type. This GMR characteristic is quite different from that of the present invention which will be described later. Thus. although the apparent film structures are similar to each other between the spin valve film in the report and that of the present invention, the essential functions of the films totally differ from each other.
In general, the magnetoresistance (MR) characteristic of the spin valve film exhibits changes as shown in FIG. 3 (described later in detail). If an exchange biasing magnetic field generated by exchange coupling at the ferromagnetic layer/antiferromagnetic layer interface between the pinned ferromagnetic layer 40 and the antiferromagnetic layer 50 and magnetic interaction between the free ferromagnetic layer and the pinned ferromagnetic layer are represented by Hex, and coercive force of the pinned ferromagnetic layer at this time is represented by Hc1, it is necessary to satisfy the condition of Hex&gt;Hc1. Unless this condition is satisfied, there is raised a problem that hysteresis is increased to cause higher noise level upon operating the element.
There has been a problem that, when the oxide antiferro magnetic layer is used for pinning the ferromagnetic layer, Hex becomes lower as compared with a case where a metal antiferromagnetic layer made of FeMn, NiMn or the like is used. Further, in order to obtain the sufficiently excellent exchange coupling at the ferromagnetic layer/antiferromagnetic layer interface, it is necessary to form the oxide antiferromagnetic layer having an excellent crystalline property. In general, in order to obtain an oxide thin film having the excellent crystalline property, its thickness should be several hundreds .ANG. or more. Therefore, when using the oxide antiferromagnetic layer for pinning the ferromagnetic layer, it is necessary to form the pinned ferromagnetic layer/the non-magnetic layer/the free ferromagnetic layer in order on the oxide antiferromagnetic layer in view of the MR element structure and the film characteristic.
However, if the oxide antiferromagnetic layer becomes too thick, the coercive force Hc1 of the pinned ferromagnetic layer increases so much that the condition of Hex&gt;Hc1 can not be satisfied. Further, there is another problem that the coercive force Hc2 and the anisotropic magnetic field of the upper-positioned free ferromagnetic layer also increase so that the high MR sensitivity can not be achieved. In an MR head as shown in FIG. 4 (described later in detail), a spin valve film (magnetic multilayered film) 200 and an insulation film 400 are disposed between shield films 300, and the recording density is determined by a shield gap interval. Accordingly, if the whole thickness of the spin valve film is increased, it is not preferable also in view of ultrahigh density recording and reproduction. Therefore, it is required that the oxide antiferromagnetic layer is as thin as possible and is still provided with the given necessary characteristic. That is, when applying the spin valve film using the oxide antiferromagnetic layer to the MR head or the like, it is necessary to form the oxide antiferromagnetic layer which can provide the sufficiently large exchange-biasing magnetic field Hex with the minimum thickness thereof.
The present invention has been made in view of the above situation, and its object is to provide the oxide antiferromagnetic layer showing practically sufficient reliability, and provide the magnetic multilayered film having a high MR ratio, a linear MR change rise-up characteristic in an extremely small magnetic field range of about -10 Oe to 10 Oe and a high magnetic field sensitivity, and further provide a magnetoresistance effect element using such a magnetic multilayered film, and a magnetoresistance device, such as a magnetoresistance effect type head, using such a magnetoresistance effect element.